In communication systems, forward error correction coding is often used in order to secure the reliability of communication. Particularly, in large-capacity backbone transmission systems, low-density parity check (LDPC) coding, turbo coding or the like having high error correction capability is used.
It is preferable to use soft decision information in order to sufficiently exhibit the performance of error correction coding such as the above LDPC coding. As the soft decision information, the log-likelihood ratio (LLR) of each bit is used. This log-likelihood ratio is an index representing the likelihood of a value for each bit.
When computing LLR in accordance with its definition, complicated computation is required. Therefore, there is a problem in that the size of a circuit that performs soft decision increases, or electric power required for soft decision increases. Consequently, contrivance for reducing the amount of processing of soft decision has been made.
As one of methods of reducing the amount of processing of soft decision, there is a method of using a look-up table. First, the LLR of each bit is computed in advance with respect to a bit sequence corresponding to each pattern of received symbols and a look-up table is created in advance. When a received signal is processed, an LLR corresponding to a received symbol obtained by modulating the received signal is read out of the look-up table, to thereby perform soft decision. For example, Patent Document 1 discloses a technique in which soft decision is performed using a look-up table, in a receiving apparatus that receives a two-dimensional modulated signal.